Respiratory Rate
Respiratory failure can become a life-threatening condition in a few minutes or be the result of a build up over several hours. Respiratory failure is very difficult to predict, and as a result continuous monitoring of respiratory activity is typically necessary in clinical, high-risk situations. Appropriate monitoring equipment can be life-saving (see Folke M, Cernerud A, Ekstrom M, Hok B; Critical Review of Non-invasive Respiratory Monitoring in Medical Care; Medical & Biological Engineering & Computing 2003, Vol 41, pp. 377-383).
Numerous studies have shown that Respiratory Rate (RR) provides one of the most accurate markers for indicating acute respiratory dysfunction, and thus is used to track the progress of patients in intensive care or post-operative care or anyone with potentially unstable respiration (see Krieger B, Feinerman D, Zaron A, Bizousky F; Continuous Noninvasive Monitoring of Respiratory Rate in Critically Ill Patients; Chest/90/5/November, 1986, pp 632-634, Browning I B, D'Alonzo G E, Tobin M J; Importance of Respiratory Rate as an Indicator of Respiratory Dysfunction in Patients with Cystic Fibrosis; Chest/97/6/June 1990, pp 1317-1321, Gravelyn T R, Weg J G; Respiratory Rate as an Indicator of Acute Respiratory Dysfunction; JAMA, Sep. 5, 1980—Vol 244, No. 10, pp 1123-1125).
RR has also been shown to be a very accurate marker for weaning outcomes for ventilated patients (see Tobin M J, Perez W, Guenther M, Semmes B J, Mador J, Allen S J, Lodato R F, Dantzker D R; The Pattern of Breathing during Successful and Unsuccessful Trials of Weaning from Mechanical Ventilation; AM Rev Respir DIS 1986; 134:1111-1118 and El-Khatib M. Jamaleddine G, Soubra R, Muallem M; Pattern of Spontaneous Breathing: Potential Marker for Weaning Outcome, Spontaneous Breathing Pattern and Weaning from Mechanical Ventilation; Intensive Care Med (2001) 27:52-68) as it exhibits high correlation with both the success and failure of extubations.
During sedation, monitoring of the RR has been shown to be a more rapid marker of the induction of anesthesia than any other clinical measure, such as lash reflex, loss of grip, cessation of finger tapping, and loss of arm tone (see Strickland T L, Drummond G B; Comparison of Pattern of Breathing with Other Measures of Induction of Anesthesia, Using Propofol, Methohexital, and Servoflurane; British Journal Of Anesthesia, 2001, Vol. 86, No. 5, pp 639-644). During conscious sedation (narcotic sedation), there is always a risk of respiratory depression. However, monitoring of the respiratory pattern combined with pulse oximetry yield the most useful information about the occurrence of respiratory depression and changes in RR typically provide an earlier warning than does pulse oximetry or end-tidal CO2 tension (see Shibutani K, Komatsu T. Ogawa T, Braatz T P, Tsuenekage T; Monitoring of Breathing Intervals in Narcotic Sedation; International Journal of Clinical Monitoring & Computing; 8: 159-162, 1991).
Respiration monitoring is also useful during non critical care, e.g. during exercise testing and different types of cardiac investigations. In the latter case there is also need to time the different phases of respiration, since the heart function is modulated by respiration. A forthcoming area of application for respiration monitoring may be that of home-care (see Hult P, et al., An improved bioacoustic method for monitoring of respiration. Technology and Health Care 2004; 12: 323-332).
Despite the obvious benefits of performing continuous respiratory monitoring, the search for an accurate, non-invasive, and non-obtrusive method to continuously monitor RR has proven to be long and unsuccessful. Several technologies have been developed in an attempt to fill this clinical gap, but none has gained sufficient physician confidence to become a standard of care. In this regard, inductive plethysmography, fiber optic humidification and capnography are among the most popular technologies. Each of these has advantages and disadvantages, but none has proven to be clearly superior. More suitable technologies are still needed to address such issues as: low signal to noise ratio, different breath sound intensities, phase duration, variable breathing patterns, interferences from non-biological sounds (electromagnetic interference, movement artifacts, environmental noise, etc.), and interference from biological sounds such as the heart seat, swallowing, coughing, vocalization, etc.
Tracheal sounds, typically heard at the suprasternal notch or at the lateral neck near the pharynx, have become of significant interest during the last decade. The tracheal sound signal is strong, covering a wider range of frequencies than lung sounds at the chest wall, has distinctly separable respiratory phases, and a close relation to airflow. Generally, the placement of a sensor over the trachea is relatively easy as there is less interference from body hair, garments, etc, as compared to chest-wall recording sites.
The generation of tracheal sounds is primarily related to turbulent air flow in upper airways, including the pharynx, glottis, and subglottic regions. Flow turbulence and jet formation at the glottis cause pressure fluctuations within the airway lumen. Sound pressure waves within the airway gas and airway wall motion are likely contributing to the vibrations that reach the neck surface and are recorded as tracheal sounds. Because the distance from the various sound sources in the upper airways to a sensor on the neck surface is relatively short and without interposition of lung tissue, tracheal sounds are often interpreted as a more pure, less filtered breath sound. Tracheal sounds have been characterized as broad spectrum noise, covering a frequency range of less than 100 Hz to more than 1500 Hz, with a sharp drop in power above a cutoff frequency of approximately 800 Hz. While the spectral shape of tracheal sounds varies widely from person to person, it is quite reproducible within the same person. This likely reflects the strong influence of individual airway anatomy.
Pulmonary clinicians are interested in tracheal sounds as early indicators of upper airway flow obstruction and as a source for quantitative as well as qualitative assessments of ventilation. Measurements of tracheal sounds provide valuable and in some cases unique information about respiratory health.
Apnea
Apnea monitoring by simple acoustical detection of tracheal sounds is an obvious application and has been successfully applied in both adults and in children. The detection of apneic events are a normal derivative from the RR estimation. A temporary cessation in breathing, typically lasting at least 10 seconds in duration, is referred to as apnea. Longer pauses may be of sufficient duration to cause a fall in the amount of oxygen in the arterial blood, and have the potential to cause permanent organ damage, or, in the extreme case, death. Adults with sleep apnea are very susceptible to exacerbation of this condition post-surgery, and therefore their respiration must be carefully monitored. Disordered breathing during sleep is a common condition with an estimated prevalence of up to 24% in men and 9% in women in North America. It is associated with excessive morbidity and increased mortality from cardiovascular and cerebrovascular events and increased risk of road traffic accidents (see Young et al., The occurrence of sleep-disordered breathing among middle-aged adults, N Engl J Med 1993; 328: 1230-1235). The condition can be suspected clinically in the presence of classic symptoms such as snoring, daytime hyper-somnolence, obesity, and male gender. The diagnosis is typically confirmed by polysomnography. The most common sleep disorder is Obstructive Sleep Apnea Syndrome (OSAS), also known as Sleep Apnea Hypopnea Syndrome (SAHS). This condition is so much linked to excessive morbidity and mortality, that it is considered a public health hazard at par with smoking (see Findley et al., Automobile accidents involving patients with obstructive sleep apnea, Am Rev Respir Dis 1988; 138: 337-340).
Heart Rate
The rhythm of the heart in terms of beats per minute may be easily estimated on the tracheal site by counting the readily identifiable heart sound waves. Heart rate (HR) is altered by cardiovascular diseases and abnormalities such as arrhythmias and conduction problems. The main cause of death in developed countries is due to cardiovascular diseases and mostly they are triggered by an arrhythmic event (ventricular tachycardia or ventricular fibrillation). The HR is controlled by specialized pacemaker cells that form the sinoatrial (SA) node located at the junction of the superior vena cava and the right atrium. The firing rate of the SA node is controlled by impulses from the autonomous and central nervous system. It is now commonly accepted that the heart sounds are not caused by valve leaflet movement per se, as earlier believed, but by vibrations of the whole cardiovascular system triggered by pressure gradients (see Rangayyan R M, Biomedica, Signal Analysis 2002, IEEE Press Series, Wiley Inter-Science). The normal (resting) HR is about 70 bpm. The HR is slower during sleep, but abnormally low HR (below 60 bpm) during activity could indicate a disorder called bradycardia. The instantaneous HR could reach values as high as 200 bpm during vigorous exercise or athletic activity; a high resting HR could be due to illness, disease, or cardiac abnormalities, and is termed tachycardia.